This invention is generally directed to electrophotography, and more specifically the present invention relates to toner and developer compositions for use in electrostatic imaging systems. In one embodiment the present invention is directed to thermotropic liquid crystalline polymers which are useful as toner compositions. These polymers are of a chemical structure enabling their melting over a narrow temperature interval wherein there is a substantial decrease in the melt viscosity at a clearing temperature which is above the melting point of the toner resin. Toner compositions having incorporated therein the thermotropic liquid crystalline polymers possess excellent flow properties, and desirable paper wetting characteristics; and moreover these toner compositions are highly useful in electrostatic imaging systems, wherein flash fusing processes are selected for enabling the adherence of the developed toner image to a supporting substrate such as paper. Additionally, toner and developer compositions with the thermotropic liquid crystalline polymers have other desirable characteristics including, for example, providing markings of high optical density and allowing matte finishes subsequent to flash fusing processing.
The development of electrostatic latent images with toner compositions comprised of a blend of toner resin particles and pigment particles is well known. Recently, there have been disclosed developer compositions with charge enhancing additives which impart negative charges, or positive charges to the toner resin particles. For example, there is disclosed in U.S. Pat. No. 4,298,672 positively charged toner compositions comprised of resin particles, and pigment particles, and as a charge enhancing additive pyridinium compounds and their hydrates of the formula as detailed in column 3, beginning at line 14. Additionally, there is disclosed in U.S. Pat. No. 4,338,390 positively charged developer compositions containing as charge enhancing additives organic sulfate, and sulfonate compositions. Illustrative examples of toner resin particles disclosed in these patents include numerous known resin compositions, such as styrene butadiene resin copolymers, styrene methacrylate copolymers, polyesters, and polyurethanes. Further, there is illustrated in U.S. Pat. No. 3,326,848 a toner composition with styrene butadiene copolymer resins, and the use of this composition for developing positively charged latent electrostatic images. Also, there is described in U.S. Pat. No. 3,960,737 a liquid developer composition containing a mixture of styrene butadiene copolymer resins and an acrylate. Moreover, disclosed in U.S. Pat. No. 3,766,072 is a developer composition with at least two types of particles, one of which is the specific styrene butadiene copolymer resin designated, Pliolite S 5-D.
Additionally, there is illustrated in U.S. Pat. No. 3,590,000 developer compositions comprised of a polyester resin, and in U.S. Pat. No. 3,900,588 developer compositions having incorporated therein minor amounts of polymeric additives, and minor amounts of abrasive materials such as colloidal silica. In U.S. Pat. No. 3,983,045, there are disclosed developer compositions wherein the toner resin particles contain solid friction reducing materials, such as zinc stearate, and nonsmearable abrasive materials, such as colloidal silica. Also, in U.S. Pat. No. 3,853,778, there is described for incorporation into toner compositions materials with an amorphous backbone and side chain crystallinity. More specifically, this patent illustrates a developer composition wherein the toner particles are comprised of a polymer selected from crystalline homopolymers, or copolymers with an amorphous backbone and side chain crystallinity derived from the polymerization of a polymerizable mixture with a polymerizable monomer having a crystalline side alkyl group therein.
It is indicated in the '778 patent that several different types of thermoplastic resins are presently used in toner particles, and while they are generally capable of producing good quality images these materials have certain deficiencies in specific areas. For example, when the toner composition is to be used on a combustible surface such as paper, some of the toners selected for fusing at a temperature which will enable sufficient adherence of the resin particles to the surface can result in charring, or burning thereof. Also, according to the disclosure of this patent, several toner resins have a very low fusing temperature causing them to be tacky at ordinarily encountered conditions, resulting in undesirable caking, or agglomeration of these particles during storage. The temperature at which caking or agglomeration occurs with a given resin is referred to as the blocking temperature for that material. Conventional toner resin materials are characterized by a blocking temperature substantially lower than the fusing temperature. Thus, a toner material having a blocking temperature substantially above the temperature normally encountered during storage, also has a high fusing temperature thereby requiring an excessively large quantity of heat energy to fuse the toner material to the copy substrate. When a high melting toner is selected for conventional xerographic apparatuses, either lower operating speeds, or larger fusers are required in order to adequately fix the deposited toner image. The heat generated by high output fusers endangers sensitive machine parts, such as selenium photoreceptors, and also can elevate room temperature to the discomfort of the machine operators. Further, in flash fusing these toner compositions are known to emit undesirable effluents, causing pollution hazards. The toner compositions of the present invention overcome many of these disadvantages.
As further indicated in the '778 patent, it is rather difficult and costly to manufacture thermoplastic resins having consistently uniform molecular weights, resulting in polymers that normally possess nonuniform melting ranges, and consequently fusing temperatures cannot in all instances be accurately predicted. Accordingly, the fusing devices selected, as indicated herein, are generally of larger then ordinary capacities. Since the temperature in the fuser should not be maintained above the char point of the paper, often it is necessary to reduce the speed at which the paper passes through the fuser unit of automatic xerographic duplicating machines. Thus, it would be desirable to formulate a toner with a balanced combination of blocking temperature, and fusing temperature, and further a toner wherein there is a desirable drop in a melt viscosity above the melting point of the toner resin particles. The toner composition of the present invention satisfies these objectives.
Moreover, there is a need for toner, and developer compositions which are simultaneously hard and tough since, for example, soft toner compositions tend to form undesirable films on reuseable photoconductive imaging members. These films which have different electrical characteristics than the photoconductive member and are hygroscopic, adversely effect the electrical conductivity of the imaging member when the imaging apparatus is operated under conditions of high humidity. However, a polymeric material which is too tough is undesirable from the standpoint of its resistance to attrition, such as by jet pulverization procedures. Also, the polymers which are hard and brittle tend to fracture when impinging upon each other, or on relatively hard machine surfaces forming undesirable fine abrasive dust particles in the toner handling apparatus, and these particles may drift in air and cause premature wearing of various machine components.
Attempts have been directed to obtaining toner resins from crystalline materials, as these polymer materials are known to melt rather sharply, rather than over a broad melting range, however, the available crystalline polymers are relatively conductive, and moreover do not generally possess a sharp decrease in the melt viscosity above the melting point of the resin. Further, many of these resins do not possess in combination desirable sharp melting points within a specific temperature interval, low polymer melt viscosity, a sharp decrease in the melt viscosity above the melting point of the toner resin, good wetting characteristics, and flow properties during fusing. The toner compositions of the present invention satisfy many of these objectives.